1. Field Of The Invention
This invention relates to computer systems and, more particularly, to methods and apparatus for providing a charge pump for generating high voltages and high currents for erasing and programming flash electrically-erasable programmable read only memory arrays (flash EEPROMs).
2. History of the Prior Art
It has been found that the use of computers has grown so extensive that the power used by these computers has become significant. In order to reduce the cost of operation as well as the consequent use of energy resources, a substantial move is underway to reduce this power usage. A major trend in the manufacture of personal computers is toward the reduction in the voltage level required to operate the integrated circuits which are used in the various components of those computers. A simultaneous trend is the desire to provide portable computers which are able to provide most of the abilities of desktop computers but are assembled in very small and light packages. This has led to attempts to reduce the power used by portable computers so that their battery life will be extended.
In order to reduce power consumption and extend battery life, much of the integrated circuitry used in personal computers is being redesigned to run at low voltage levels. This reduces the power usage and allows more components to be placed closer to one another in the circuitry. The circuitry and components used in portable computers are being designed to operate at voltages levels such as 5 volts and 3.3 volts. This helps a great deal to reduce the power needs of personal computers.
However, at the same time, the desire to offer more features in portable computers opposes this salutary result. Many of the features require higher voltages to function. For example, one real convenience is the ability to change the basic input/output and startup (BIOS) processes as improvements in a computer or its peripherals occur. Historically, this has been accomplished by removing the electrically programmable read only memory (EPROM) or similar circuitry providing the read only memory for storing the BIOS processes and replacing it with new circuitry at additional cost. This is a complicated operation beyond the abilities of many computer users. Recently, flash electrically-erasable programmable read only memory (flash EEPROM memory) has been used to store BIOS processes. This memory may be reprogrammed without removing the BIOS circuitry from the computer by running a small update program when the BIOS processes are changed. However, reprogramming flash EEPROM memory requires approximately twelve volts to accomplish effectively; and the lower voltage batteries provided in personal computers are not capable of programming and erasing flash EEPROM memory.
Another form of flash EEPROM memory array provides another example of high voltage requirements in portable computers. Recently, a new form of long term random access storage has been devised using flash EEPROM memory arrays. An example of a flash EEPROM memory array which may be used in place of a hard disk drive is given in U.S. patent application Ser. No. 07/969,131, entitled A Method and Circuitry For A Solid State Memory Disk, S. Wells, filed Oct. 31, 1992, and assigned to the assignee of the present invention. These arrays provide a smaller lighter functional equivalent of a hard disk drive which operates more rapidly and is not as sensitive to physical damage. Such memory arrays are especially useful in portable computers where space is at a premium and weight is extremely important. However, these flash EEPROM memory arrays also require much higher voltages for writing and erasing data than can be provided directly by the batteries of portable computers. In situations in which batteries do not provide sufficient voltages, it has been typical to provide charge pumps to generate higher voltage from the lower voltages available. However, although such voltages pumps are able to increase the voltage to an appropriate level, prior art charge pumps do not provide sufficient current to generate the power for effectively erasing and programming flash EEPROM memory without the use of very large capacitors which utilize an inordinate amount of die space.